Application of specific electrical energy to the spinal cord for the purpose of managing pain has been actively practiced since the 1960s. While a precise understanding of the interaction between the applied electrical energy and the nervous tissue is not fully appreciated, it is known that application of an electrical field to spinal nervous tissue can effectively mask certain types of pain transmitted from regions of the body associated with the stimulated nervous tissue. More specifically, applying particularized electrical pulses to the spinal cord associated with regions of the body afflicted with chronic pain can induce paresthesia, or a subjective sensation of numbness or tingling, in the afflicted bodily regions. This paresthesia can effectively inhibit the transmission of non-acute pain sensations to the brain.
Electrical energy, similar to that used to inhibit pain perception, may also be used to manage the symptoms of various motor disorders, for example, tremor, dystonia, spasticity, and the like. Motor spinal nervous tissue, or nervous tissue from ventral nerve roots, transmits muscle/motor control signals Sensory spinal nervous tissue, or nervous tissue from dorsal nerve roots, transmit pain signals. Corresponding dorsal and ventral nerve roots depart the spinal cord “separately”; however, immediately thereafter, the nervous tissue of the dorsal and ventral nerve roots are mixed, or intertwined. Accordingly, electrical stimulation intended to manage/control one condition (for example, pain) often results in the inadvertent interference with nerve transmission pathways in adjacent nervous tissue (for example, motor nerves).
As illustrated in FIG. 1, prior art spinal column or spinal cord stimulators (SCS) commonly deliver electrical energy to the spinal cord through an elongate paddle 5 or epidural electrode array containing electrodes 6 positioned external to the spinal cord dura layer 32. The spinal cord dura layer 32 surrounds the spinal cord 13 and is filled with cerebral spinal fluid (CSF). The spinal cord 13 is a continuous body and three spinal levels 14 of the spinal cord 13 are illustrated. For purposes of illustration, spinal levels 14 are sub-sections of the spinal cord 13 depicting that portion where the dorsal and ventral roots join the spinal cord 13. The peripheral nerve 44 divides into the dorsal root 42 and dorsal root ganglion 40 and the ventral nerve root 41 each of which feed into the spinal cord 13. An ascending pathway 92 is illustrated between level 2 and level 1 and a descending pathway 94 is illustrated from level 2 to level 3. Spinal levels 14 can correspond to the vertebral levels of the spine commonly used to describe the vertebral bodies of the spine. For simplicity, each level illustrates the nerves of only one side and a normal anatomical configuration would have similar nerves illustrated in the side of the spinal cord 13 directly adjacent the paddle 5.
Typically, SCS are placed in the spinal epidural space. Conventional SCS systems are described in numerous patents. Additional details of the placement and use of SCS can be found, for example, in U.S. Pat. No. 6,319,241 which is incorporated herein by reference in its entirety. In general, the paddle 5 is about 8 mm wide and from 24 to 60 mm long depending upon how many spinal levels are stimulated. The illustrated electrode paddle 5 is adapted to conventionally stimulate all three spinal levels 14. These exemplary levels 1, 2 and 3 could be anywhere along the spinal cord 13. Positioning a stimulation paddle 5 in this manner results in the electrodes 6 spanning a plurality of nerves, here the dorsal root ganglion 40, the ventral root 41 and peripheral nerve 41 on multiple spinal levels.
Because the paddle 5 spans several levels the generated stimulation energy 8 stimulates or is applied to more than one type of nerve tissue on more than one level. Moreover, these and other conventional, non-specific stimulation systems also apply stimulation energy to the spinal cord and to other neural tissue beyond the intended stimulation targets. As used herein, non-specific stimulation refers to the fact that the stimulation energy is provided to all spinal levels including the nerves and the spinal cord generally and indiscriminately. Even if the epidural electrode is reduced in size to simply stimulate only one level, that electrode will apply stimulation energy indiscriminately to everything (i.e., all nerve fibers and other tissues) within the range of the applied energy 8. Moreover, larger epidural electrode arrays may alter cerebral spinal fluid (CSF) flow thus further altering local neural excitability states.
Another challenge confronting conventional neurostimulation systems is that since epidural electrodes must apply energy across a wide variety of tissues and fluids (i.e., CSF fluid amount varies along the spine as does pia matter thickness) the amount of stimulation energy needed to provide the desired amount of neurostimulation is difficult to precisely control. As such, increasing amounts of energy may be required to ensure sufficient stimulation energy reaches the desired stimulation area. However, as applied stimulation energy increases so too increases the likelihood of deleterious damage or stimulation of surrounding tissue, structures or neural pathways.
To achieve stimulation the targeted tissue, the applied electrical energy should be properly defined and undesired energy application to non-targeted tissue be reduced or avoided. An improperly defined electric field may not only be ineffective in controlling/managing the desired condition(s) but may also inadvertently interfere with the proper neural pathways of adjacent spinal nervous tissue. Accordingly, a need exists for stimulation methods and systems that enable more precise delivery of stimulation energy.